Is it permissible for me to leave school before the official end of the school day, following the example of my colleagues, and is it permissible for me not to abide by student evaluation regulations if I deem them ineffective or unjust?

If the custom has been established for the teacher to leave after the end of their classes, and the administration is aware of this, then there is no harm in acting upon it. As for student evaluation policies, any policy that is unjust is not permissible to act upon, due to the Hadith: "O My servants, I have forbidden injustice for Myself and made it forbidden among you, so do not commit injustice against one another." As for what contains no injustice, you must adhere to it, and you can discuss with the administration to rectify the policies.